This invention relates to the separation of selected particulate material from mixtures of liquids and solids. More particularly, the invention is directed to a method, an apparatus and a reagent used in the method and apparatus to effect the desired separation.
There are numerous known methods and systems for separating the selected particulate material from mixtures of liquids and solids. In many instances, this involves the simple classification of solid materials according to their densities as they flow through a vessel. U.S. Pat. Nos. 763,662, 805,382, 1,156,041, 1,425,187, 1,578,274, 1,593,232, 1,772,386, 2,198,915, 2,492,936 and 2,660,305 are deemed to represent the state of the art with respect to the effecting of separation of selected particulate materials from a mixture of liquids and solids. These patents represent the closest prior art related to the substance of the invention disclosed herein.
Various types of reagents, bath flotation systems, and froth flotation processes using various types of reagents are generally disclosed in these patents. The U.S. Pat. No. 1,578,274 discloses a method of treating particles of matter for separating same in an environmental condition which maintains the material under substantially nonoxidizing conditions. However, the specific reagents and the manner in which this is effected within a particular vessel, is completely different from the method and apparatus as disclosed herein. The complex apparatus is avoided through the use of the apparatus as disclosed herein. Furthermore, this particular prior art flotation process is completely different from that as set forth herein.
The various other prior art reagents disclosed in these U.S. patents operate in the presence of high oxygen environmental conditions, both from a chemical and a mechanical standpoint. Mechanically, the bubbling of air and the establishing of turbulence within the flotation zone aggravates the environmental flow conditions in which the selected particulate material is to be separated. Consequently, the mixture is subjected to oxygen. The presence of oxygen in the system establishes an oxygen-controlled surface condition on the particulate material selected for separation. Turbulence also upsets any desired intermolecular activity with respect to the estabishment of electrostatic charges on the selected particulate material. Chemically, the reagents include oxygen which becomes available to react unfavorably with respect to the surface of the selected particulate material.
The U.S. Pat. No. 2,660,305 discloses an apparatus for classifying solid materials according to their various densities. The solids in this particular prior art method and apparatus are disposed in a liquid flowing in an open channel. The laws of gravity will cause the various solids having different specific gravities to collect in various layers or stratum locations. Thus, the materials flowing in an open channel will line up in accordance with their specific gravities. There are significant amounts of turbulence in such a system, while establishing different rates of flow at varying heights of the open channel. This prior art methodology is referred to as lamellar flow used to separate solids according to their densities.
The physics of the most common sedimentation process--the settling of solid particles from fluid media--has long been known. The settling velocity equation formulated in 1851 by G. G. Stokes is the classic starting point for any discussion of the sedimentation process. Stokes showed that the terminal settling velocity of spheres in a fluid was directly proportional to the difference in densities of fluid and solid, the radius of spheres involved, and the force of gravity; and inversely proportional to the viscosity of the fluid. Stokes' equation is valid, however, only for spheres of very small size and, hence, various modifications of Stokes' Law have been proposed for nonspherical particles and particles of larger size.
No settling velocity equation, no matter how valid, provides a sufficient explanation of even the basic physical properties of natural sediments. The grain size of the classic elements and their sorting, the shape and roundness of these elements and their fabric and packing are a few parameters in complex processes. These parameters are related not only to the density and viscosity of the fluid medium, but also to the velocity of the forward motion of the depositing fluid and to the turbulence resulting from this motion, the roughness of the beds over which it moves, to various mechanical properties of the solid material propelled, to the time or duration of the transport action, etc.
The sedimentary rocks associated with coal are accumulated sediments. Their constituents are of varied nature and their composition depends on the relative proportions of the materials of diverse origins. Those consisting mainly of the products of abrasion ("rock flour") and the washed residues of weathering (sand, silt, clay) are the clastic sediments. Those consisting mainly of the chemically or biochemically precipitated materials (calcium carbonate and silica) are nonclastic materials or sediments. Not uncommonly, the sediment has a multiple origin and is, in truth, a hybrid deposit. Usually the original composition is modified by reaction with materials in solution in the medium from which it was deposited or with those in solution in the ground waters with which it may later come in contact.
In sedimentary rock analysis, as many as 25 constituents may be determined and recorded. The most abundant or major constitutents are stated as oxides, and any rock analysis of good quality records the content of at least nine or ten of these. They are silica, alumina, ferric and ferrous oxides, magnesia, lime, soda, potash and water. These rock elements occur typically as oxides, simple silicates, aluminates, fluorides, chlorides, and sulfides in either anhydrous or hydrated forms.
Pyrite is a naturally occurring disulfide. Pure pyrite contains 46.67% iron and 53.33% sulfur. The hardness of pyrite is 6.0-6.5 and the specific gravity about 5. Pyrite is very common in vein deposits with other sulfide minerals and quartz. It is common in sedimentary rocks, such as shale, coal and limestone.
An analysis of coal preparation provides a basis for explaining the various aspects of the invention disclosed herein. The conditions of coal separation relate generally to the whole of selected particulate separation in a liquid medium.
There are all kinds of associations of coal and rock ranging in an unbroken series from pure coal through coal with an increasingly higher proportion of inorganic impurities, continuing through rocks having a more or less carbonaceous content and finishing with pure rock. Accordingly, the specific gravities of these bodies form an unbroken series between the two extremes represented by pure coal and pure rock. Thus, in practice, the problem is to separate impure coal from more or less carbonaceous rock, and the arbitrary demarcation line is dependent on economic and experimental considerations.
Coal, as it appears in the ground, is never free from impurities. Some of the impurity is deemed "inherent", i.e. denoting impurity derived from the original substance of the plants and animals of which the coal is composed or impurity so dispersed in the coal that it cannot be separated by cleaning processes and can be seen only with the aid of a microscope. Larger impurities are pyrite (FeS.sub.2) and marcasite (also FeS.sub.2). Another impurity is in the thin beds called "partings," and occupies crevices or faults cutting through the coal at sharp angles, and is known as shale (slate), calcite (CaCO.sub.3), sandstone, clay and roof rock.
With the onset of mechanical mining, run of mine coal became smaller and dirtier. In many mines more than a quarter of the coal brought to the surface was rejected as waste, and, thus, grew the mountainous heaps of rock and coal which disfigure coal mining areas.
The specific gravity of coal depends in some degree upon its intrinsic impurities, and ranges between 1.2 and 1.5. Most of the extraneous impurities mined with the coal are much heavier than the coal itself, and separation of coarse coal can be effected by dense media methods.
All concentrating table cleaning involves considerable loss of coal, and is generally not considered efficient relative to capital cost and space requirements.
Very small sizes of run of mine coal can be cleaned by oil flotation methods. Coal to which much dirt adheres may be crushed to minus 50 mesh and fed into a bath of water through which air is bubbled. Oil, as a collector, spreads as a film over the surface of the coal particles. The oil-covered particles attract themselves to the entrained air bubbles and are carried to the surface of the bath. The rock particles, however, are preferentially wetted by water and not by oil, and so sink to the bottom of the bath. The froth on the surface of the bath is then removed and broken down to release the clean coal. See "Flotation Study of Refractory Coals," by Kenneth J. Miller, U.S. Bureau of Mines Report #8224.
Most substances are capable of existing in at least three forms or states of aggregation; namely, the solid state, the liquid state and the vapor state. Water is a familiar example. These forms, differing in internal structure and in physical properties, such as density, mobility, refractive index and heat content, constitute various phases of the substance. If a substance exhibits more than one crystalline form, as does sulfur, each of its polymorphic varieties is a distinct solid phase of the substance. The number of possible phases in a system containing more than one substance may include solutions in both the liquid and solid states and other phases that are, in general, chemical compounds resulting from the chemical combination and interaction of the components.
The materials of nature and of technology are either homogeneous (monophasic) or heterogeneous (polyphasic) systems. Ice is a single phase. A sample of water consisting of some liquid with its vapor is in two phases. A piece of granite generally contains three phases, being essentially a conglomerate of pieces of quartz, mica and feldspar. Each phase, although it may be present in separate pieces, is considered to be one homogeneous kind of matter. The phase is distinct in structure, though not always in chemical composition, from the other phases of the heterogeneous sample and is mechanically separable from them.
The conditions for the coexistence at equilibrium of various combinations of the possible phases of a system and for their formation and transformation, as controlled by variations in pressure, temperature and composition, constitute the subject of phase equilibriums. The principles of phase equilibriums find basic applications in practical problems of extractive metallurgy, such as disclosed herein, and, in general, in problems of physical and chemical separation.
When molecules come near enough to one another to influence each other, at least two forces are brought into play; one of attraction and one of repulsion. If molecules did not exert forces of attraction, they would not cohere, as they manifestly do in the liquid and crystalline states. Were there no forces of repulsion, the forces of attraction would be supreme, and nothing would prevent molecules from annihilating one another. Much as human conduct is determined by a conflict of loyalties, molecular behavior is, to a large extent, determined by the balance struck between the forces that tend to pull molecules together and those that tend to push them apart.
When two ions have charges of the same sign, their mutual energy and the force acting between them are positive, denoting repulsion. When the ions have charges of opposite signs, their mutual energy and the force acting between them are negative, denoting attraction. It is this force that accounts for the tenacity with which electrons in atoms are held to the positively charged nuclei.
Surface phenomena is that phenomena occurring at boundaries between phases of matter, such as between a solid and a liquid. Physical effects of surface phenomena include surface tension, adsorption, thin films, electrical double layers, wetting, adhesion, etc. These, in turn, bear on detergency, waterproofing, friction, flotation, corrosion, and electrode reactions. Stability of dispersions (colloidal), of emulsions, and of foams depends largely on surface properties.
In hydrocarbons, the interactions are weak and are due to dispersion forces resulting from momentary dissymetries in electron clouds about the atoms. Dispersion forces also contribute to this interaction in the case of liquids having stronger intermolecular attractions, such as water and alcohol. Such interactions are hydrocarbon bonds. The interaction between an interior carbon atom in diamond and each of four neighboring carbon atoms is like that between the two carbon atoms in ethane, H.sub.2 C--CH.sub.3 (covalent bonds). The intermolecular forces holding liquids or crystals of hydrocarbons, water, alcohols or similar materials together--whether dispersion forces, dipole forces, orhydrogen bonds--are collectively characterized as weak.
Special types of adsorption are found at solid/liquid interfaces. Ions may be adsorbed by the solid, imparting a charge to the solid. This phenomenon is essential to the stability of most colloidal systems. Suitable organic ions can be adsorbed on particle surfaces to "waterproof" selectively wherein wet particles will settle out or separate from particles adsorbing the organic ions.
A special class of surface phenomena is furnished by films of long chain polar organic molecules on a water surface. The molecules referred to are such that a long hydrocarbon chain, usually containing from 14 to 30 carbon atoms, is attached to a small polar group such as carboxylic acid or alcohol group; stearic acid (C.sub.17 H.sub.35 COOH) is representative.
The long hydrocarbon chain renders the solubility of the material in water negligible. However, the polar group is attracted to water and, consequently, the material spreads readily on water to form a unimolecular film. The surface tension of the film-covered water is less than that of pure water. Therefore, a barrier placed in the path of the spreading film will experience a force which can be considered as a net "pull" in the direction away from the film, or as a "push" by the film. The difference between surface tensions of pure and film-covered water is called the "spreading pressure" of the film.
An electrical double layer results at the interface between the two phases when one phase is charged relative to the other. One phase may become charged relative to the other by adsorption of ions. Whether the charge on the surface is produced by electrons or ions, ions of opposite charge are drawn toward the surface by it, forming an atmosphere of countercharge near the surface. The combination of charge and countercharge is called the electrical double layer. Electrokinetic phenomenon and the stability of lyophobic sols, foams and emulsions are ultimately related to the structure of the electrical double layer.
The orientation of molecules adsorbed at an interface strongly determines wetting properties. Also, adhesion of two dissimilar materials to each other is closely related to wetting. From a molecular point of view, a joint interface will adhere strongly to each other if they have molecules which can interact strongly. Adhesives contain polar molecules or groups; thus, it is possible to secure good adhesion with polar surfaces such as metallic and nonmetallic compounds. On the other hand, it is difficult to secure good adhesion to nonpolar solids, such as coal.
An understanding in basic fluid mechanics is deemed important to an understanding of the invention.
Many flow phenomena are so complicated that a purely mathematical solution is impossible, incomplete or impractical. Thus, it is necessary to resort to experimental meaurements. Dimensional analysis and dynamic similarities are two tools which have proved helpful in the organization, correlation and interpretation of experimental data. Dimensional analysis is a mathematical method useful in determining a convenient arrangement of variables in a physical relation and in planning systematic experiments. The first step is to list all the variables involved. This step may be the result of judgment or experience.
The actual dimensional analysis can be made by following a formal procedure:
______________________________________ F = MLT.sup.-2 or M = FT.sup.2 L.sup.-1 L = length (dimensional) M = mass (dimensional) F = force (dimensional) T = time (dimensional) L.sup.2 = area (dimensional) L.sup.3 = volume (dimensional) LT.sup.-1 = linear velocity (dimensional) LT.sup.-2 = acceleration (dimensional) Force = mass .times. acceleration = dimensional relation ______________________________________
Buckingham's theorem can be used to organize variables in the smallest number of significant groups.
Open channel flow is more empirical than that of other aspects of fluid mechanics such as that of pipe flow. In order to obtain information about the flow around or through a structure called the prototype, it is often convenient and economical to experiment with a model of the prototype. Tests with models provide an advantage in research and design which cannot be obtained from theoretical calculations alone.
Dimensional analysis coefficients:
(a) Reynold's number PA1 (b) Froude's number PA1 (c) Weber's number PA1 (d) other dimensionless numbers (other force ratios could be devised depending on the forces determining the particular flow)
Some variables:
(1) Flow pattern (magnitude and direction of velocity and acceleration) over, through or around the object. PA0 (2) Pressure distribution and resulting forces on the object or its parts. PA0 (3) Flow capacities and calibration of the various flow passages. PA0 (4) Energy loss due to shear drag and pressure drag. PA0 (5) Molecular properties at surfaces of separation; and reagent action.